The present invention relates to a flexible electronic device for mounting on a curved or flexible support and a method for fabrication of the same.
The ability to fabricate flexible electronic devices is becoming increasingly desirable owing to the trend of incorporating electronics in a growing variety of applications. For example, these devices may be employed in xe2x80x9csmartcardsxe2x80x9d, that is, credit card sized pieces of plastic including microcontrollers and secure memory.
Large area electronics (LAE) made on flexible plastic substrates have been found to suffer from a number of problems. These include low thermal budgets, poor quality devices, cracking and poor adhesion of layers in the semiconductor devices, take up of water, and poor layer to layer alignment due to expansion and contraction of the plastic substrate.
WO-A-00/46854 describes a technique for producing flexible active matrix displays. Shaped blocks carrying circuit elements are deposited in a slurry onto a flexible substrate which includes complementary recesses. The blocks drop into respective recesses and are then electrically coupled together to form an active matrix.
In a process disclosed in EP-A-1014452, thin film devices are formed on a separation layer which is provided on a substrate. Hydrogen ions are implanted into the separation layer. The separation layer is then parted from the substrate by irradiating the layer with laser light, this process being accelerated by the effects of the prior implantation step. The devices can then be transferred to another substrate. A large-scale active matrix substrate may be formed by the transfer of a plurality of smaller units that have been fabricated separately on other substrates.
An object of the present invention is to provide an improved method of forming an electronic device which is able to flex and an improved flexible electronic device.
The present invention provides a method of fabricating an electronic device comprising the steps of:
(a) forming a predetermined pattern of weakened regions in a layer of rigid material which define contiguous portions of the rigid layer;
(b) providing electronic components on the rigid layer; and
(c) forming flexible connectors which extend between components on different portions.
According to the method, the components and connectors are conveniently formed on a rigid layer and the weakened regions ensure that subsequent flexing of the device divides the rigid layer in a predetermined manner. The flexible connectors extend across the weakened regions so that the connections are maintained once the rigid layer has been divided along the weakened regions, allowing further flexing of the device without affecting the device""s operation. Where the weakened regions are provided on the opposite surface of the rigid layer to the components and connectors, it may be preferable to carry out step (a) after step (b) and/or step (c). The rigid layer is then in a more robust form during the process of fabricating the components and/or connectors.
The method enables a circuit of substantial area to be transferred in toto to a flexible substrate and then to be fractured in a controlled manner so that the combination is flexible. The rigid layer may be fractured in the finished device, or alternatively, the breakage may occur during use of the device.
The invention further provides a method of fabricating an electronic device comprising the steps of:
(a) providing electronic components on a rigid layer;
(b) forming flexible connectors which extend between components on different contiguous portions of the rigid layer; and
(c) dividing the rigid layer into the contiguous portions.
The invention combines the handling advantages of fabricating electronic components and connectors on a more robust, brittle material with the ability to flex the device after the components and connectors have been formed. In a preferred embodiment, the method includes the step of mounting the rigid layer over a flexible substrate. The flexible substrate may provide support to the rigid layer once it has been divided, without preventing flexing of the device.
The method is applicable to the formation of large area electronic (LAE) devices on glass or other rigid materials, or the production of integrated circuits on silicon, for example.
The present invention also provides an electronic device comprising a layer of rigid material having electronic components thereon, contiguous portions of the rigid layer being defined by weakened regions of the rigid layer, and flexible connectors extending between components on different portions.
The invention additionally provides such a device in which the rigid layer has been divided along one or more of the weakened regions such that the device is flexible. The weakened regions may comprise grooves in one or both faces of the rigid layer.
According to another aspect, the invention provides an electronic device comprising a layer of rigid material having electronic components thereon, and flexible connectors extending between components on different contiguous portions of the rigid layer, the rigid layer being divided into the contiguous portions such that the device is flexible.
Thus, the electronic device is provided with the contiguous portions already separated, rather than being joined by weakened regions. This may be appropriate where the device is sufficiently robust in divided form for the following handling thereof. The connectors alone may be secure and strong enough to hold the contiguous portions together until the device is mounted on a substrate.
The rigid layer preferably comprises a brittle material such as glass or silicon, whilst the flexible layer may comprise plastic, for example.
The weakened regions may be formed by etching or sandblasting the rigid layer, or using a diamond circular saw blade. Alternatively, they may be formed by scribing the rigid layer, using a diamond tipped cutter or a diamond or carbide edged wheel for example, or by using a laser to ablate a fine groove or slot. The weakened regions may comprise grooves or slots which extend part-way through the layer from one or both faces. In other embodiments, they may comprise slots or perforations that pass completely through the rigid layer.
In further preferred embodiments, the weakened regions may comprise fine grooves or slots formed in the base of relatively wide grooves. The greater width and depth of a wider groove may reduce the separation occurring at the opposing end of the fracture in the rigid layer, when it is flexed such that the angle defined by the rigid layer surfaces adjacent to the groove is reduced. Also, the depth of the wider groove may result in the rigid layer fracturing more easily. The fine groove ensures that the location of the fracture at the weakened region is precisely defined.
The weakened regions are preferably linear, that is they extend along straight and/or curved lines, and also preferably have a width substantially less than their length, so that breakage occurs in a controlled manner along their length. The weakened regions may be created before or after the electronic components have been fabricated on the rigid layer. The contiguous portions defined by the weakened regions should be sufficiently small that further breakage of the rigid layer beyond the fracture of the weakened regions does not occur during normal use of the device.
The rigid layer may be divided into contiguous portions by using a laser or other energy beam to locally heat the layer, followed by a rapid cooling or xe2x80x9cquenchingxe2x80x9d process using an air jet or liquid coolant for example. The stress induced by this process may cleanly fracture the layer without the need to flex it. In other cases, for example with a relatively thick layer, such a process may merely weaken the layer in predetermined regions, enabling it to be fractured by subsequent flexing.
The connectors may for example be formed in step (d) by electroplating metal onto the rigid layer. This may include the step of depositing a seed layer prior to electroplating the metal connectors. Areas of photoresist may be defined over the rigid layer prior to electroplating the metal, such that portions of the connectors form bridges over the photoresist, and the photoresist is subsequently removed. The resulting connector is therefore able to flex, thereby maintaining electrical connection of components on different portions despite flexing of the device. Each connector may comprise a single flexible bridge portion that extends over the location where the weakened portion of rigid layer is to fracture, or it may comprise several bridges in a concertina-like structure.